The present invention relates to a device for controlling a servo-assisted brake for a motor-vehicle, in particular a parking brake, of the type comprising an actuator unit arranged to apply a tension force on at least one brake operating cable and a force sensing unit adapted to provide a signal indicative of the force applied on the operating cable.
A control device of this type is disclosed in International Patent Application WO 98/56633. This control device comprises an electric motor which drives, through a gear reduction unit, a male-and-female screw mechanism, in which the screw is rigidly connected to an actuating member so as to cause this latter to move axially. An end of the operating cable is mounted on the actuating member through a pack of springs, in such a manner that it can move axially relative to the actuating member against the resilient biasing action exerted by the spring pack. A force sensing unit for detecting directly the force exerted on the operating cable by the actuating member is associated to the actuating member. The sensing unit comprises a movement sensor which senses the relative movement between the actuating member and the operating cable due to the compression of the spring pack.
The main shortcoming of this device is represented by the loss of operability in case of failure of the force sensing unit, since this latter is directly mounted on the drive chain between the operating cable and the screw-type actuator.
A further example of a control device of the above-specified type is known from International Patent Application WO 02/57122. This known control device comprises an electric motor which drives, through a gear reduction unit, the rotation of a screw meshing with an axially movable nut screw. The operating cable is attached at an end to the nut screw so as to be pulled or released by the latter. The screw is mounted axially floating and is pulled, as a result of the force exerted on operating cable and therefore on the nut screw, against a spring interposed between the end of the screw facing the cable and an abutment surface of the device. A position sensor senses the compression of the spring and enables to establish the force exerted by the control device on the operating cable.
Compared to the prior art discussed above, this control device provides for a force sensing unit uncoupled from the operating cable, since it is not directly interposed between the cable and the driving screw. However, this device has the shortcoming of considerable axial size. In fact, in order for the control device to be able to operate also in case of failure of the force sensing unit (that is to say, of the spring on which the screw acts), the length of both the threaded portion of the screw and the portion of the screw on which the output gear wheel of the gear reduction unit is fitted has to be increased by an amount equal to the travel which would result from a possible breakage, amount which might even be equal to the overall rest length of the spring.